Finishing and Insulating garage walls

[Drewski123]

The garage is right underneath the house with one exterior wall (concrete wall about 6 feet tall; tested the concrete sidewall for moisture, no moisture found. There is also a window on the side of the concrete wall. Additionally, there are two interior walls (2x4 stud walls, 16 inches on center) that separate the conditioned living spaces from the garage.
The garage is unheated (semi-conditioned, the temperature never gets below 40 degrees in the winter) with a heating duct running to the above bedroom.

Should I build a wall and insulate the concrete wall? If yes, what are the proper layers? I will not be planning on heating the garage.

In regards to the other two interior walls, should I use any vapor barriers with the insulation? If yes, what are the proper layers?

I will be using Type X drywall with Ultra Touch Cotton Bats.

I live in Seattle, in the marine zone 4.

Thanks for you help,
Andy

[jadnashua]

I think I'd glue some foam insulating panels to the concrete then install some drywall on top of it (foam requires a fire resistant barrier on top of it) rather than building a wall. If you want to install shelves or something similar on that wall, you'd need much longer anchors and likely a batten board to distribute the weight.

[hj]

You ALWAYS install the vapor barrier on the heated side of the insulation. If you are not heating the garage I see absolutely no benefit to insulating the concrete wall. The garage will always revert to ambient/outdoor temperature because of the garage door.

[Dana]

Don't use fiber insulation only on below-grade concrete walls with or without vapor barriers or you risk mold. When you insulate the wall, the temperature of the concrete will drop, and will be below the dew point of the garage space air at least part of the time, but if you put a garage-side vapor barrier/retarder up the humidity inside the studwall will rise to the sub-soil dampness. If it's dry enough in your area that you can bury untreated wood without risk of rot you'd be OK, but that's a much drier soil condition than you'll ever find in Seattle.

The solution is to put a vapor-retardent insulating layer between the susceptible wood and the concrete, which would be rigid-foam. In a Seattle climate even R4-R5 of foam between a 2x4 studwall would keep the batts and wood at non mold-inducing humidity year round, whether you actively heat the garage or not. But you still must avoid vapor barriers on the interior side to keep ground moisture from accumulating the batts & studs. A mere 1" of XPS (R5) or EPS (R4) would get you there in your climate. Since the wall tests as low-dampness you can even use foil or poly faced goods if you like. (If the wall showed higher humidity there could be some risk to the sill plate atop the wall, especially if there is no sill gasket.)

Simply gluing the foam to the wall and putting gypsum over it would not meet fire code, since the glue isn't sufficiently secure under fire conditions (or over the long term.) If you go with an all-foam solution, use 1x furring over the foam (through-screwed to the concrete 24" o.c. with TapCons) on which to mount the gypsum.

The sub-soil temp in Seattle is about 50F, and an uninsulated wall under the house represents a very real heat load to the total house (unlike hj's Cave Creek AZ location). Insulated garage doors with reasonable weatherstripping are available, an would be useful here, but even with an uninsulated door, with insulate exterior walls the garage temperature will track the condition space temperature more closely than the outdoor ambient temp.

[Drewski123]

Thanks everyone. I've decided not to insulate the exterior concrete wall, however I will be replacing the two interior walls and a ceiling with Type X drywall (the interior walls and the ceiling are currently covered with 6" wide floor planks which do not meet the code).

Knowing that the average temperature in the garage stays above 45 degrees, and knowing that I will be heating the living spaces on the other side of the walls (currently I don not heat the rooms, they are used as storage spaces, but I am planning to finish the basement in the future), should I add any vapor barrier to the stud walls? I was planning to add R13 UltraTouch bats to each stud wall and cover it with 5/8" Type X Drywall and apply some latex paint. Should this be sufficient?

[Dana]

If I understand it correctly, the walls in question have conditioned or semi-conditioned space on one side, the garage space on the other. In that case putting a vapor retarder on the garage side would be counterproductive, limiting the ability of the assembly to dry.

In a Seattle climate putting a vapor retarder on the interior/conditioned-space side of the wall wouldn't hurt, but isn't necessary- the garage space is a vented space between the exterior of the interior wall with the weather-resistant layers to the outdoors. It's a HUGE vented space for the wall to dry into, with zero chance of weather-related wetting on the garage side.

DO insulate the exterior concrete when you do the build-out finishing the garage though.